The present invention relates to an amplifier with an integrated resistance-capacitance feedback network, and more particularly to such an amplifier having a field effect transistor (FET) for use at microwave frequencies.
FETs, especially those having high power capability, have a low input impedance at microwave frequencies, such as X or K bands, due to their gate input capacitance and can be unstable at certain combinations of source and load impedances, thereby making a broadband impedance match difficult to achieve. In order to reduce these problems negative feedback around the FET is used. When the feedback is implemented in hybrid circuit form, bond wires are used, which form is expensive due to the labor involved, does not result in repeatable characteristics, is mechanically unreliable, occupies a large area, and has parasitic inductances and couplings. Further, the feedback network exhibits transmission line effects, which result in unstable operation at high frequencies where the feedback signal can have undesirable phase shifts. Also, phase shifts can occur in the gate pad between different sections of a high power FET, so the output voltages from the sections do not add in phase; this reduces power output as compared to the situation when all sections add in phase. The feedback network and FET have been combined in a prior art monolithic circuit construction, in which all external connections except for input and output leads are eliminated. The input and output leads are on opposite sides of the integrated circuit package, and the feedback network has been connected between the input and output leads using metallization runs. This results in a more reliable circuit, and reduces the occupied space and cost. While the transmission line effect, phase shift effect, and parasitic couplings and inductance are reduced they are not eliminated using these construction procedures, however, the inventors observed. Accordingly the inventors sought a monolithic circuit construction avoiding metallization runs that tend to cause these undesirable effects at high frequencies.